This invention relates to satellite communications.and more particularly relates to such communications employing optical beams, such as laser beams.
The beams used for space-to-space and space-to-ground optical communications have extremely narrow beam widths that require high bandwidth, closed loop control for pointing and tracking to maintain adequate signal power for communications. The beam widths are so narrow (on the order of 1-20 microradians) that methods are needed to initially acquire the communications beams from the usual 0.1-0.3 degree pointing knowledge uncertainty of current spacecraft. The acquisition method must be highly robust and minimize total weight and power requirements for the optical communications terminal.
Beam acquisition methods have been described in the past. For example, in columns 9-11 and FIG. 5, U.S. Pat. No. 3,504,182 (Pizzurro et al., issued Mar. 31, 1970) describes an acquisition method in which a first beam of a first satellite dwells at one point in a field of view while a second beam of a second satellite scans the entire field of view. When the beams illuminate their respective satellites, the acquisition terminates.
U.S. Pat. No. 3,511,998 (Smokler, issued May 12, 1970) describes an acquisition method employing slow oscillatory scan motion limited by limit switches. Receipt of a second beam signal during the slow scan motion terminates the acquisition (Column 11).
U.S. Pat. No. 5,060,304 (Solinsky, issued Oct. 22, 1991) describes an acquisition method relying on beam reflection (Abstract).
U.S. Pat. No. 5,282,073 (Defour, et al., issued Jan. 25, 1994) describes an acquisition method in which the width of the beam is altered during acquisition (Columns 5-6).
U.S. Pat. No. 5,475,520 (Wissinger, issued Dec. 12, 1995) describes an acquisition method in which multiple transmitted beams are defocused to provide wide area coverage during acquisition (Column 2).
U.S. Pat. No. 5,592,320 (Wissinger, issued Jan. 7, 1997) describes an acquisition method in which a beam is modulated with time or location information during the acquisition (Column 3).
U.S. Pat. No. 5,710,652 (Bloom et al., issued Jan. 20, 1998) describes an acquisition system employing an array of a CCD acquisition camera (Column 5).
Each of these prior methods and systems have limitations which decrease its usefulness.
The invention is useful in a communication system employing an optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. In such an application of the invention, the beam is transmitted from the first terminal for alignment with a beam receptor located on the second terminal. According to a preferred embodiment, the beam is first generated. The beam then is transmitted toward the second terminal, preferably by optics. During transmission, the beam is scanned over a controlled uncertainty region defining an outer perimeter beginning at a starting scan point with a first scan pattern and continuing at another scan point with a second scan pattern different from the first scan pattern. The scanning preferably is conducted by a positioning mechanism and a controller.
By using the foregoing techniques, terminal weight and power can be minimized and the design of the positioning mechanism is simplified, because.the degree of acceleration required at the central portion of the scan is reduced.